Can not fetch text from next page content

First of all, thanks for this simple n yet great plugin. It works like a charm.

I only have one issue here, i.e. fetching one line text from next page content for teaser purpose. When I used manual –nextpage– tag, the code can fetch text from next page content . But when –nextpage– tag is generated automatically by your plugin, the code can not fetch it, thus it returns null.

Did I miss something? Here’s the code to fetch text for teaser:

if ($page < $numpages) {
  $nextpage = intval($page) + 1;
  $next_link = get_permalink() . '/' . $nextpage;	

  $content = get_post_field('post_content', get_the_ID());
  $content_parts = explode('<! --nextpage-- >', $content);
  $teaser = strip_tags($content_parts[$page]);
  $teaser = substr($teaser, 0, 30). "...";

   echo '<p>'. $next_link . $teaser . '</p>';
}

thanks for your help